ensure links only belong to 1 list

also some small documentation changes.

tests/test_linkedlist.c

@@ -57,6 +57,10 @@ int main (void)
         t->data3 = rand() % UINT64_MAX;
         t->data4 = rand() % UINT64_MAX;
 
+        // ensure our link points at nothing.
+        t->link.next = NULL;
+        t->link.prev = NULL;
+
         ni_list__push_back(&data_instances, &(t->link));
 
         printf("write: n = %d: %ld %ld %ld %ld \n\tnext: %p\n\tprev: %p\n",
@@ -75,6 +79,7 @@ int main (void)
         }
     }
 
+      // drain from back of list and remove then free entries
       while (!ni_list__is_empty(&data_instances)) {
         ni_list_node* tail = ni_list__get_back(&data_instances);
         ni_list__remove(&data_instances, tail);